An unknown person is SOMEONE who is not known or recognized, and can refer to a variety of people. The term 'ALIEN' (5 letters) is used to describe SOMEONE who is not from the same place or culture as the speaker, while 'SOMEONE' (7 letters) and 'SOMEBODY' (8 letters) are more general terms for an unidentified individual. 'STRANGER' (8 letters) is another word for an unknown person, usually implying that the person is unfamiliar or not well-known.